Timewalker weekend events are definitely my favorite - it's such a joy to be able to go back into these old dungeons I used to run years ago and play them in a way that, though perhaps not a serious challenge, still requires you to do more than just auto-attack each enemy once.
This is a feature in its infancy, and Blizzard has indicated that this is really the trial run for an expansion of the feature. What might we see in the future?
More Dungeons from More Eras:
BC and Wrath are the expansions people most often think back on as the "golden age" of WoW, which is I'm sure why they chose to make the first timewalker dungeons a selection from these expansions. While there are some (mainly Oculus) that they might hold back on due to a lack of popularity, I do hope they open up more of these dungeons as time goes on.
Beyond these, though, I'd hope that they could open up later-expansion dungeons like Magister's Terrace and Forge of Souls. This would be somewhat trickier to do - you have to take into account that people will have secondary stats that are allowing for much higher crit, haste, and such than the "you just hit the level cap" level that they lower us to for the existing ones. People would be more greatly incentivized to put together a dedicated timewalking set for these dungeons to maximize their best stats, but those late-expansion dungeons are among my favorites in the game.
Obviously, the next step is to open up Cataclysm and Mists of Pandaria dungeons for Timewalking. The former were tuned to be quite difficult at the time, but considering how the Burning Crusade ones don't seem too hard now, I imagine that the Cataclysm ones won't need too much tweaking. The Mists ones I think will just need some time for them to feel truly old-school. Likewise, I wouldn't be surprised if they hold off on making timewalker Warlords dungeons until after Legion - to give us some distance to allow us to develop nostalgia for them.
And then there's vanilla, of course. This one's tricker, as someone expecting a 20-40 minute 5-boss instance when they queue is probably not going to be happy if they get Blackrock Depths. Still, you can definitely find some more modern-style dungeons among the 1.0 dungeons. Of course, some of these were revamped for Cataclysm and other expansions - though it might be cool to have the older versions of them available for Timewalking. Still, scaling would be tricker here, as there wasn't a standardized heroic mode at the level cap, so each dungeon from vanilla would have to scale us down to different levels and item levels. Also, given that we keep all our abilities, I don't even know if you could scale anyone down low enough for a place like Blackfathom Deeps to still be challenging.
This one's tricky. On one hand, I'd love to be able to do Timewalkers all the time, but on the other, it's actually probably for the best that it's a limited, periodic thing. The current expansion's dungeons are meant to be the ones you're more incentivized to run, and it would be bad if Timewalkers totally took over the place of contemporary dungeons.
Instead, I'd extend the "weekend event" to be a "week-long event." Some players can only play certain days of the week, and it's annoying that they would be locked out of these.
Given that Legion will not have Apexis Crystals (though they could have something similar,) it's clear that there's going to be some kind of shake-up for weekend events. Adding a few segments to the rotation shouldn't be too hard, so you could even set it up that there's always a Timewalker event going on, it's just the era that changes.
As a feature that came in at the end of Warlords, it was pretty easy for them to put the rewards for Timewalkers in a kind of useful catch-up position. The 660/675 loot is fantastic for someone who just hit the level cap, and really blows the 630 stuff from heroics out of the water, allowing you to jump into Tanaan Jungle, raid finder, and even Mythic dungeons (if you get a lot of drops) fairly quickly.
But as a feature that will presumably exist at the launch of Legion, there's a real question of where the loot should fit on the scale.
This would all be a lot easier if we still had something like Justice and Valor points - you could simply have Legion heroics reward more than Timewalkers, but have it scale up as there were new tiers of content.
The other option, and something that will be coming in 6.2.2, the patch formerly known as 6.2.1, is to add a bunch of cosmetic rewards. I think this is a great idea, and one that is easily expandable. Timewalkers are primarily there for the fun of running those old dungeons and getting your nostalgia kicks, so they don't really need to be tied so tightly to progression.
The key, however, is to ensure that there are compelling cosmetic rewards that people can continue to work toward for a long time - really it means that they have to keep coming up with more rewards. This of course will overlap a bit with the Darkmoon Faire - the ultimate "not for power, just for fun" feature of the game, but I think you can come up with enough thematic differences to make them feel different enough.